I just upgraded to Firefox 4.0.1 and noticed that I had the issue, too. Safari and Chrome, however, played the same clips without a problem.

Another issue with Hotmail prompted me to restart Firefox in 32-bit mode. To my surprise, this seems to have resolved the YouTube-issue as well. So, I'm guessing that running Firefox in 32-bit mode might solve it.

If you're using MacOSX, go to your Applications folder, right-click on the Firefox icon and select "get info", and then check the "start in 32-bit mode"-checkbox. I'm not sure how it works on Windows, though.

Unfortunatly this ain't much of use for me. I'm on WinXP and there is no such option. also, i uncheked the "enable hardware acceleration" option in flash player settings. didnt help out much, though. what else can i do? it's get annoying and started today. (yesterday everything worked out great.) additional info: when i right click on the square where there should be movie, it shows 2 options: with gray it says "movie cannot be loaded" (or similiar, translated from my language) and "adobe flash player 10.xxxx.xx - informations".